Touch screens have been widely used as input interfaces for electronic devices for detecting touch inputs within a display area. A touch screen often includes a touch panel combined with a display screen. A touch panel usually has a construction based on a matrix of sensor nodes that form a two-dimensional array in a grid. For example, as shown in FIG. 1, a capacitive touch panel is often made of horizontal conductive lines (e.g., the horizontal conductive line 102) and vertical conductive lines (e.g., the vertical conductive line 104) which do not contact each other. Cross points of the horizontal conductive lines and the vertical conductive lines correspond to sensor nodes (e.g., the sensor node 106). When the touch panel is operated, an electric field is formed between two conductive lines at a sensor node. A finger touching the panel blocks some of the electric field, therefore reducing the capacitance at the sensor node. Each sensor node may be checked at each sampling interval to obtain capacitance measurement data which is processed to generate a detection signal. The detection signal is then compared against a predetermined threshold to determine whether the sensor node is touched. For an increasing number of applications, multiple simultaneous touches on a touch screen are to be detected. For example, it is often needed for a touch screen to detect gestures, such as a pinching motion between a thumb and a forefinger of a user.
Capacitive measurements for touch detection includes self capacitance sensing and mutual capacitance sensing. FIG. 2(A) and FIG. 2(B) depict example diagrams showing self capacitance sensing for a touch panel. Electrodes 1002 and 1004 are formed on an insulating material 1006, and correspond to one or more conductive lines (e.g., the conductive line 102, the conductive line 104). As shown in FIG. 2(A), the electrode 1004 is connected to ground, and a stimulus signal 1008 (e.g., Tx, an alternate-voltage signal) is applied on the electrode 1002. Without any touch event, a self capacitance associated with the touch panel is CS. When a finger 1008 touches the touch panel as shown in FIG. 2(B), the capacitance between the finger 1008 and the Earth ground is CBody, and the capacitance between a device ground (GND) of the touch panel and the Earth ground is CBoard. Both CBody and CBoard are sufficiently large, and the finger 1008 can be considered as a virtual ground. The self capacitance associated with the touch panel changes to (CS+CF), where CF represents a capacitance between the finger 1008 and the insulating material 1006.
FIG. 3(A) and FIG. 3(B) depict example diagrams showing mutual capacitance sensing for a touch panel. Electrodes 1102 and 1104 are formed on an insulating material 1106. A stimulus signal 1108 (e.g., Tx, an alternate-voltage signal) is applied on the electrode 1102, and a response signal 1110 (e.g., Rx) is received at the electrode 1104. Without any touch event, a mutual capacitance between the electrode 1102 and the electrode 1104 is Cm. When a finger 1118 touches the touch panel as shown in FIG. 3(B), the capacitance between the finger 1118 and the Earth ground is CBody, and the capacitance between a device ground (GND) of the touch panel and the Earth ground is CBoard. Both CBody and CBoard are sufficiently large, and the finger 1118 can be considered as a virtual ground. The mutual capacitance associated with the touch panel decreases in magnitude in response to the finger touch. Thus, a touch event on a touch panel can be identified by detecting the change of the self capacitance or the mutual capacitance associated with the touch panel.